1. Field of the Invention
This invention relates to a thermosettable polyphenylene oxide (PPO) plastisol dispersion composition which, on heating at or above the fluxing temperature but below the flow temperature of the PPO, rapidly provides handling strength through plasticization and which can be crosslinked to give a thermoset on further heating or irradiation.
The invention also relates to a process for forming a crosslinked polymeric material and fiber reinforced prepregs and composites.
A polymeric material containing the following recurring structural units: ##STR1## wherein n is much greater than m, will both be referred to herein as poly(phenylene oxide) (PPO).
2. Description of Prior Art
In general, a plastisol is composed of a high molecular weight polymer dispersed in a plasticizer which is a material incorporated in a plastic to increase its workability. Upon heating, the plastisol turns to a pregelled dispersion, to a gelled dispersion and then to a fused dispersion. The viscosity of a plastisol decreases with the increase of temperature at the beginning. At a certain temperature, suddenly, the viscosity increases sharply and the liquid dispersion turns to an opaque solid, a gelled dispersion. This temperature is called minimum fluxing temperature defined as the temperature at which a plastisol develops sufficient physical integrity to permit being lifted from the fusion plate. Upon further heating at a higher temperature, the plastisol turns to a clear plasticized plastic.
To prepare a plastisol, two basic ingredients, a high molecular weight polymer powder and a liquid plasticizer, are required to form a stable dispersion after blending. Physically, the plasticization process of a plastisol is the permeation of the plasticizer into the polymer particle to solvate the polymer molecules. The permeation rate (P) depends on the diffusion speed of the plasticizer (D) and the interaction between the plasticizer and the polymer (S): EQU P=D.times.S
Under the assumption that a polymer is compatible with a certain plasticizer, two important factors, the molecular weight (size) and the structure of polymer and plasticizer, should contribute to the stability of a plastisol which is determined by the diffusibility of the plasticizer upon aging. A stable dispersion should not allow the diffusion to occur at or below storage temperature. To prevent a plasticizer from diffusing, the size of the plasticizer molecules has to be larger than that of the polymer free volume. Upon heating, the free volume increases with temperature and allows the plasticizer molecule to diffuse into a polymer particle when the temperature is high enough.
Besides the kinetic process of plasticization, the capability of plasticization also depends on thermodynamic parameters. The plasticization should not occur when the free energy of mixing is greater than or equal to zero (.DELTA.G.sub.m .gtoreq.O), even if the size of the plasticizer is as ideal as described above.
Poly(vinyl chloride) and its copolymers, because of their low degradation temperature, eliminate hydrogen chloride and form a colored product below their melting temperature. Therefore, to use a melt process for these polymers without adding a plasticizer is difficult. The invention of plastisol technology has allowed these polymers to have excellent applicability and become the major polymers used in the plastisol industry. Unfortunately, the degradation of these polymers in service conditions is still an unacceptable problem in some applications due to hydrogen chloride elimination which promotes corrosion in metal and a reduction of polymer strength.
To stabilize PVC plastisols in service and to enhance their service life, a crosslinkable, secondary plasticizer has been incorporated with a primary plasticizer for plastisol preparation. The secondary crosslinkable plasticizer includes reactive vinyl compounds such as trimethylolpropane trimethacrylate and tetraethylene glycol dimethacrylate [Dainippon, JP80 52,335 (1980)]; G. F. Cowperthwalte, SPE Journal, 29, 56, 1973], unsaturated polyesters [Dainippon, JP80 21,474 (1980)], diallyl compounds [Shin-Nippon Rika, JP72 40,853 (1972)], and epoxy resins [Dunlop, JP81 100,841 (1981)].
To further improve the structural properties and eliminate the problem of hydrogen chloride release of the PVC plastisol, the plastisol technology was extended to acrylic polymers for the preparation of thermally fusible acrylic plastisols. See U.S. Pat. No. 4,125,700, which used esters as plasticizers and polyol acrylates as reactive diluents to prepare various reactive acrylic plastisols which formed a plasticized semi-interpenetrating network after the crosslinking reactions.
U.S. Pat. No. 4,020,966 also teaches a plastisol composition containing as a resin component a copolymer of a normal alpha-olefin and maleic anhydride in combination with a plasticizer and a reactive polyepoxide plasticizer.
The use of epoxy for making thermosetting epoxypoly(phenylene oxide) blend has been reported. However, to disperse a powdered poly(phenylene oxide) in a thermosetting resin for making a reactive plastisol, which can be plasticized into a continuous solid material by the thermosetting resin at a temperature well below the flowing temperature of the polymer, is novel. The plasticized solid material becomes a thermoset after the crosslinking reaction.
Using solution casting, a mixture of poly(oxy-1.4-phenylene), epoxy resin and epoxy curing agent dissolved in an organic solvent such as toluene, a mixture of methyl ethyl ketone and toluene, or methylene chloride, was cast into film and the phase separation phenomenon studied after curing [British Polymer J., 15 (1), 71-5 (1983)]. The solution was also used to impregnate glass and Nomex fiber fabrics for making composites. [JP77 30978; JP77 53973, JP82 151624; JP83 69052, JP83 69046; JP83 84844]. This process involved the use of an organic solvent and caused an emission problem. The removal of solvent in film casting increases the cost of the process due to the expensive solvent and energy consumption. Furthermore, the time required for removing the solvent is another concern of production efficiency. Therefore, a high solid thermosetting composition is a superior choice rather than solvent casting.
German Offenlegungsschrift No. 21 62 894 teaches the use of epoxy resin-poly(phenylene oxide)-glass fiber blend for injection molding application at melt condition (330.degree. C.). In the patent, the blend contains no curing agent because of the potential problem of the crosslinking reaction of epoxy resin at the extrusion temperature. Using a curing agent in the process would produce a cured epoxy material in the extruder before injection. A similar approach was also described in Japanese Patent No. J73039017.
This invention relates to the application of the plasticization principle, originally developed for PVC plastisols, to prepare non-PVC reactive plastisols from poly(phenylene oxide). The advantages of this approach include: (1) the elimination of an organic solvent; (2) a fast B-stage formation through plasticization at a temperature much lower than the melt temperature of the polymer; and (3) the presence of a crosslinking agent without the risk of curing at the processing temperature.
To prepare a pumpable thermosetting material, the instant invention discloses the dispersion of PPO powder in a reactive plasticizer such as liquid epoxy resin, liquid acrylic resin or the mixture of the two. This invention concerns the preparation of an ideal reactive plastisol which is a system containing a non-reactive polymer powder dispersed in a thermosetting plasticizer. The ideal reactive plastisol converts to a fused plastisol at minimum fluxing temperature, turns to a clear plasticized polymer at clear point and changes to a thermoset material after the crosslinking reaction. The characteristics of the reactive plasticizer thus include: wide compatibility with the polymer; low vapor pressure; high plasticization efficiency; excellent aging stability upon storage and crosslinkability upon curing.
This invention particularly concerns a class of reactive plastisols prepared from a PPO polymer powder and a reactive plasticizer or a combination of reactive plasticizers.
In reactive plastisol technology, which is a combination of plastisol and thermosetting technologies, the dispersion fuses into a plasticized solid at a temperature much lower than the melting point. Because the fluxing process is extremely quick, the thermosettable material provides a handling strength or B-stage strength in a few seconds. The final cure to a thermoset material can then be made to occur either by subsequent heating to the cure temperature or by irradiation, e.g., UV in the presence of a photoinitiator or by high energy ionizing radiation.